Sweetening process



Dwz, 1947. D. E; PAYNE Em.-

SWEETENING PR'ocE'ss Filed Dec. 31, 1945 Patented Dec. 2, 1947 UNITEDSTATES PATENT OFFICE sWEETeeeEs-S Donald E. Payne, Chicago, ildvadetveeVonrl hees, Homewood, Ill., assignors vto Standard Oil Company, Chicago,Ill., a corporation of Indiana Appiieation Deeember 31, 1942, serialV o.516,368

(o1. ieee-s2) 5 Claims. 1

This invention relates to a process of sweetening hydrocarbon oils andparticularly petroleum distillates such as gasoline and kerosene. Theprocess is especially applicable to the swcetening of cracked productssuch as gasoline and kerosene `from thermal or catalytic crackingprocesses. One of the objects of the invention is to provide a processof removing mercaptans from gasoline and other light petroleum productsby extraction with alkaline solutions.` Another object of the inventionis to improve the alkaline extraction of mercaptans by employing a newmethod of regenerating the alkaline solutions which are recycled in theprocess. Still another object of the invention is to provide a methodfor producing sweet gasoline which will pass the doctor test without theuse of expensive reagents or retreating operations. In brief, oui`invention, W'l-iich will be described more specically hereinafter,involves the use of a second stage of regeneration for at least a partof the caustic solution used in sweeten-ing, after the solution has beenregenerated incompletely in the usual manner, and it involves moreparticularly the extraction' of the partially regenerated caustic with astream of sweetened distillate from the process itself.

The process of sweetening gasoline involves primarily removal from thegasoline of mercaptans which are dissolved therein in a small amount. Ithas heretofore been demonstrated that the mercaptans of low molecularweight such as ethyl and propyl mercaptans are relatively easily removedby extraction with alkaline solutions such as caustic soda or causticpotash. The higher molecular weight mercaptans such as amyl and hex-ylmercaptans, being more soluble in the oil, are less readily removed bycaustic solutions. To facilitate the removal of mercaptans it hasheretofore been the practice to add to the caustic solution an organicreagent or solvent commonly called a solutizer which is soluble in thecaustic, thereby increasing the solubility of the mercaptans in thecaustic solution and facilitating their extraction from the gasoline.From 5 to l% of solutizer is usually eiective, and the amount ofsolution required to sweeten gasoline is" usually 2 to 20%, for example4 to 10% by vollime based on gasoline treated.

As the extraction proceeds, for example in a continuous process wherethe gasoline is passed thru a body of the caustic solution or where thegasoline is continuously extracted in a tower thru which causticsolution is recirculated from a supeA ply, the mercaptan content of thecaustic soon irlioielss t' poit Where micaptans are Ilo longercompletely removed from the gasoline, at leastno't suiicient to providea sweet product which will. pass the doctor test. This test has longbeen used the' `petrolouin industry and consists in shaking a sample ofoil with a al-` kaline solution of sodium plunfibte in the presence oi atrace of sulfur. IiD the oil contains an ob eotionable lan'iciiint ofm'ercaptan, Va colored precipitate of lead meroaptide Will `be observed.

In ord-ei" to obtain more complete removal of mercaptans with alkalinesolutions, either with cr the addition of organic mercaptan solvent-s,it is necessary to regenerate the alkaline solution, by' which is' meantthe removal of merc-ptiisi therefrom; Ina-y be accomplished severalways,- -one of the inost common methods being to' Heat the solution toan elevated temperdiz-ure, for example to the boiling point or nearboiling point, and vaporiz'e oi the mercaptans by a current of air,steam or an inert gas, such as -ue gasa* This regen-eration technique isbased oii the fact that' the alkaliriiercaptides, forexample sodium orpotassiur'n mercaptides, contaidi th alkaline Solution dissociate athigher temperatures as the Vapor pressure of the meroaptan eduilibriuitherewith increases. By removing`- the 'iercaptan vapor, the equilibriumis displaced and the' caustic solution may be sube stanti-ally freed ofmercaptans. This solution is then .cooled and returned to the gasolineextractor for" further us.- The mercaptan vapors liberated from` such'-a process may be condensed aridthe rnecaptans recovered, or the vaporsmay lie-burned in a furnace.

Another method of regeneration which has certain advantages from thestandpoint of saving oI llieat :comprises contacting .themercaptancontaining alkaline solution with air or oxygen or otheroxidizing agent at normal temperatures or ternperature'sA only slightlyelevated, for example to' 130 F. The mercaptans are oxidized by the airin the presence of caustic to organic disul'ii'd'es which are no longersoluble in the caustic solutionandwhich may be separated therefrom,after'wliich the alkaline solution is returned to the extractor forfurther use. The oxidation of mercaptans to disuldes is not a very rapidreactiomparticularly in the case of the lower molecular weightmercaptallns,` andtherefore it is desirable to employ a catalyst tospeed up this reaction. A number of catalysts have been proposed andused heretofore, both organic and inorgzmic.` ,In the latter class arethe salts of lead, copper, cobalt. nickel, cadmium, zinc and vanadium,while the organic oxidation catalysts are generally phenolic incharacter including cresols, amino phenols, and particularly thepolyhydroxy aromatic compounds, such as hydroquinone, catechol,pyrogallol, dihydroxynaphthalene and the various tannins. Thesecatalysts are generally effective in amounts of 0.1% up to 2 or 3%.

When using the organic regeneration catalysts, diculty is sometimesencountered with resinication or overoxidation of the catalyst as aresult of long contact with air or oxygen in the regenerator. This mayoccur, for example, when using tannin as the catalyst in which case itis desirable to avoid regenerating the caustic solution with air to apoint where complete regeneration is obtained. As a result, a smallamount of mercaptan may be returned to the gasoline extractor andincomplete sweetening of the gasoline follows, owing to theimpossibility of completely removing mercaptans from gasoline with acaustic solution which contains mercaptans. Even when not confrontedwith the problem of regeneration catalyst destruction,rthere is alwayssome diiculty in producing sweet gasoline by regeneration and recyclingalkaline solutions owing to the diilculty of regenerating thesesolutions to a point Where the mercaptan concentration will besuiciently low to provide the necessary equilibrium for sweet gasoline.In order to pass the doctor test a gasoline must contain less than about.001% of mercaptan sulfur.

In our process we have provided a method of obtaining sweet gasoline bysubstantially completely removing the mercaptans from at least a part ofthe caustic solution after the solution has been regenerated in theusual manner and before recycling back to the gasoline extractor. Onemodication of our process is illustrated by the accompanying drawingwhich shows diagrammatically an apparatus for carrying out the process.

Referring to the drawing, a hydrocarbon feed stock, for example gasolinewhich may be cracked or straight-run gasoline, or an absorption napthais charged to the process by line I leading to pretreating extractor I Iwherein the gasoline is contacted with an alkaline solution introducedat I2. The principal purpose of the pretreatment in Il is to remove fromthe gasoline any hydrogen sulfide it may contain and otheracidicmaterials such as naphthenic acids and phenols. On account oftheir high oil solubility the phenols are not completely removed byalkaline washing but their concentration is reduced to a point wherethey do not interfere with subsequent operation of the process. For thispurpose We may use caustic soda, caustic potash, sodium carbonate, orother convenient alkaline` material. When washing cracked gasoline, asodium hydroxide solution of to 30%, for example 20%, is satisfactory.The caustic for the purpose may be stored in supply tank I3 and recycledthru lines I2 and I4. Additional caustic may be supplied to the systemby line I5 and spent caustic may be withdrawn by line I5. The spentcaustic solution, which may contain acids and phenols dissolved thereinin an amount of about to 40%, preferably 25 to 30%, may be acidiedsubsequentlyto recover phenols which are of value as insecticides andwood preserving agents.

The washed gasoline from pretreater II is conducted by line II to themain mercaptan extractor I8. Tower I8 may be provided with suitablebaile plates or packed with broken tile, Raschig rings, Berl saddles, orother suitable contacting material. As the gasoline flows upward thruthe tower I8 it encounters streams of caustic solution introduced at I9,20 and 2| before it passes out at the top of the tower thru line 22sufciently depleted of mercaptans to meet the desired specifications.The amount of caustic solution can be about 5 to 15%, for example. Forthe purpose of our process, caustic solutions containing from about 10to 30% are preferred, but other concentrations may be used, e. g. 5 to40%, an example of a suitable concentration being 20%. In generalcaustic potash is somewhat more effective for mercaptan removal thancaustic soda. The caustic solution is withdrawn from the base of theextractor by line 23 and conducted thru heater 24 to regenerator 25.Tower 25 may also be a baffled or packed tower, or a mechanicallyagitated vessel, in which the caustic solution is contacted with acurrent of air introduced at 26 and vented at 2. At the base of thetower the solution is drawn into settling section 28 where disuldes areallowed to separate and are withdrawn by line 29. Gas oil or other lowvolatile solvent may be introduced into separator 28 to assist inremoval of disulfides. Catalyst for regeneration which may consist of anaqueous or alkaline solution of catechol or tannin is introduced by line30 in the desired amount. After having once been brought up to thedesired concentration it is not necessary to add additional catalystexcept at infrequent intervals to make up for mechanical losses due toleakage and entrainment and slight losses from chemical destruction ofthe catalyst due to side reactions.

The partially regenerated caustic solution, which may be regenerated,for example, to the extent of to 98%, is Withdrawn by line 3l andrecycled thru cooler 32 and line I9 to the extractor. It is preferred tooperate the extractor at ordinary temperature, for example 30 to 50 inwinter and up t0 70 or 90 in summer, while the caustic regenerator 25 ispreferably operated at a temperature of around to 130 F., although atemperature of F. may be used. We may operate our regenerator at thesame temperature as the mercaptan extractor, however. Inasmuch as thepartially regenerated caustic in line I9 contains mercaptans, it willnot produce a sweet gasoline at that point in the extractor but servesto remove the bulk of the mercaptans in the gasoline and leave thereinonly a small residual amount. In order to reduce the mercaptanconcentration still further, We withdraw a portion of the partiallyregenerated caustic solution by line 33 and conduct it to causticpurifier 34 where it is extracted by a stream of sweet gasolinewithdrawn from line 22 by line 35 leading thru heat exchanger 36 andheater 36a. The amount of gasoline withdrawn to purier 34 may be about10 to 50% of the gasoline charged to extractor I8, preferably about 20to 30%. In purifier 34 the small amount of mercaptans remaining in thecaustic are transferred counter-currently to the gasoline which is thenconducted by line 3'I back thru exchanger 36 and cooler 36D to anintermediate point in the extractor I8. The purified caustic solutionnow substantially free of mercaptans is conducted by line 38 thru cooler39 and line 20 back to the extractor, introducing it at a pointsubstantially above the point of introduction of the partiallyregenerated caustic solution previously introduced at I9. The uppersection of the extractor therefore forms a second extraction zone and ifdesired, separate towers may be used for the purpose.

In case separate towers are used, itis preferred to conduct thealkalineso'lution from the bottom of the second extraction zone to thetop of the first extraction zone but it may also be conducted directlybac-k to the regenerator 25.

In operating our process, we prefer to maintain the temperature of thecaustic purifier 34 above the temperature of the extractor I8 in orderto obtain a favorable equilibrium for transfer of mercaptans fromgasoline to caustic in I8 and caustic to gasoline in 34. For thispurpose, a temperature differential of about 25 lto 150 F. is usuallysuiiicient, preferably about 100 F. The caustic purifier 34 may beoperated, if desired, at temperatures in the range of 150 to 225 F. inorder to obtain more co-mplete removal of mercaptans from the causticsolution. For this purpose suitable heating coils may be supplied in 34and the caustic solution in line 33 may be diverted thru heater te. Whenthe temperature is above the boiling point of the distillate, pressurewill be needed.

The proportion of the caustic solution in line 3l which is diverted thruline 33 for the purpose of completing the sweetening at the top ofextractor I does not need to be rigidly defined as there is considerableflexibility in the operation of our process. Thus when a fairly completeregeneration of caustic solution is obtained in 25, we may divert only 5or 10% of the caustic solution thru purifier 313. Under otherconditions, we may divert 35 to 50% of the caustic solution thru thepurifier and we may even operate with all the caustic solution beingconducted thru the purier 34, depending on the economics of theoperation, a factor affected by many variables. In general we prefer todivert about 10 to 25% of the caustic thru the purier.

On acc-punt of unavoidable contamination it is sometimes 'desirable towithdraw caustic solution from the system and replace it with freshcaustic, and this may be done by line il and discharge valve d2. Ifdesired, the caustic removed from the system in this way may be employedin the pretreating system by conducting it thru valve 43. Fresh causticsupplied thru line 2l may be of the same strength as that used in thesystem or it may have a higher strength if it is desired to build up theconcentration of the caustic in the system.

By introducing the fresh causticl at a point above the point ofintroduction of regenerated caustic thru line 2G we obtain additionalextraction of mercaptans by means of caustic solution which is entirelyfree of mercaptans, thus providing a still more favorable equilibriumfor mercaptan removal than that obtained at the lower levels in theextractor I8 by solutions introduced at I9 and 20. If desired, a portionof the caustic solution regenerated in 25 may be completely stripped ofmercaptans by steaming, in apparatus not shown, and this solution may becontinuously supplied to the extractor by line 2| in order to maintainthe desired differential in mercaptan concentration required in theprocess. When operating in this way, extractor I8 and caustic purifier34 may be operated at the same temperature, eliminating the need forheat exchangers, heaters and coolers.

As indicated above, We may employ various organic reagents or solventsin the alkaline solution to increase the solubility of mercaptanstherein. Examples of such solvents are the phenols and alkyl phenols,cresols, ethyl and propyl phenols, xylenols, organic acids, naphthenicacids, fatty acids, preferably of 3 to 6 carbon atoms, furoic' acid,alcohols, preferably those 4of low volatility such a glycol andglycerine, ethanol and propanol amine, and the alkyl substituted benzoicacids. The solvents are usually effective in concentrations of about 5to 50%, depending on solubility and on the character of the solvent.Phenols contained naturally in gasoline, particularly cracked gasoline,may be permitted to collect in the caustic alkali solution to aneffective amount, e. g. 10 to 25%, and the amount may be controlled byadding phenols recovered from gasoline or by regulating the amount ofphenols removed in the prewash stage. The extracting solution can beprepared for example by starting with an approximately 30% caustic sodasolution and operating on an alkyl phenol containing charging stock suchas a cracked gasoline until about 2 mols of the caustic have beenconverted to sodium phenolate or cresylates, giving a washing solutioncontaining about 20-25% of phenolates and 20% free caustic. The additionof about 1% of catechol, hydrcquinone, or tannin to this solution issuicient to enable it to be regenerated and recycled almostindefinitely, make-up being necessary only for mechanical losses.

Having thus described our invention what we claim is:

l. The process of sweetening mercaptan-containing petroleum distillateswhich comprises contacting said distillates countercurrently in a firstextraction Zone with an aqueous caustic alkali solution containing asolutizer for mercaptans, separating said caustic alkali solution anddissolved mercaptans from partially sweetened distillate and contactingit with air in a first regeneration stage in the presence of a phenolicmercaptan oxidation catalyst, thereby partially regenerating thesolution by oxidation, controlling the oxidation to convert a majorproportion but not all of said mercaptans to organic disulfidesinsoluble in said caustic solution while avoiding destruction of saidphenolic oxidation catalyst by excessive oxidation, recycling a portionof said partially regenerated caustic solution to said first extractionzone and contacting it with petroleum distillate to remove mercaptansfrom additional amounts of distillate supplied thereto, countercurrentlycontacting another portion of said partially regenerated causticsolution in a second regeneration stage with a stream of sweetdistillate produced in the process, the volume of which is substantiallyless than the volume of mercaptancontaining distillate undergoingtreatment, thereby transferring residual mercaptans from said partiallyregenerated caustic solution to said distillate stream and producing acompletely regenerated caustic solution, separating said distillate andcaustic solution in said second regeneration stage, recycling theseparated distillate stream to said first extraction zone and combiningit therein with the mercaptan-containing distillate first mentioned,countercurrently contacting in a second extraction zone said completelyregenerated caustic solution with said partially sweetened distillatefrom said first extraction zone, thereby producing a sweet distillateproduct, separating said product from caustic alkali solution in saidsecond extraction zone and transferring the lastmentioned solution tosaid rst extraction zone wherein it is contacted with saidmercaptan-containing distillate rst mentioned.

2. The process of claim 1 wherein the second regeneration Stags isoperated at a higher tem- 7 perature than the temperature of saiddistillate extraction zones.

3. The process of claim 1 wherein hydrogen sulfide, phenols and organicacids are separated from the distillate charged to said first extractionzone by initially contacting it with an alkaline solution.

4. The process of sweetening a petroleum distillate which comprisescontacting a continuous stream of said distillate with an aqueouscaustic alkali solution in two successive extraction zones, partiallysweetening said distillate in the rst of said extraction zones whereinmercaptans are dissolved in said alkali solution, separatingmercaptan-containing alkali solution from distillate in said firstextraction Zone and subjecting it to oxidation by contacting with oxygenin a rst regeneration zone at a temperature of about 120 to 150 F.,regulating the extent of oxidation to convert a major part of thedissolved mercaptans in said solution to disulfides while retaining aminor part of the mercaptans unoxidized, separating disuldes in saidfirst regeneration zone and discarding them from the system, recycling apart of the partially regenerated alkali solution from said firstregeneration zone to said rst extraction Zone for extraction therein ofadditional amounts of sour distillate, transferring another part of thepartially regenerated alkali solution from said first regeneration Zoneto a second regeneration zone wherein substantially all remainingmercaptans are removed by countercurrent contacting in the absence ofoxygen with a substantially mercaptan-free liquid hydrocarbon stream,thereby transferring mercaptans from said solution to said hydrocarbonstream, the resulting hydrocarbon stream containing mercaptans beingthen charged to said first extraction Zone for further removal ofmercaptans therein, conducting regenerated alkali solution from saidsecond regeneration Zone to the second of said distillate extractionzones, and contacting it therein with partially sweetened distillatefrom said first extraction zone, thereby completely sweetening saiddistillate, separating alkali solution from distillate in said secondextraction zone and combining it with the alkali solution in said rstextraction zone for the removal of mercaptans from said additionalamounts of sour distillate therein.

5. The process of sweetening a petroleum distillate which comprisescontacting a continuous stream of said distillate with an aqueouscaustic alkali solution in two successive extraction Zones, partiallysweetening said distillate in the iirst of said extraction zones whereinmercaptans are dissolved in said alkali solution, separatingmercaptan-containing alkali solution from distillate in said firstextraction zone and subjecting it to oxidation by contacting with oxygenin a rst regeneration zone at a temperature of about 120 to 150 F.,regulating the extent of oxidation to convert a major part of thedissolved mercaptans in said solution to disulfides while retaining aminor part of the mercaptans unoxidized, separating disulfides in saidrst regeneration Zone and discarding them from the system, recycling a,part of the partially regenerated alkali solution from said rstregeneration zone to said first extraction zone for extraction thereinof additional amounts of sour distillate, transferring another part ofthe partially regenerated alkali solution from said rst regenerationzone to a second regeneration zone wherein substantially all remainingmercaptans are removed by extraction in the absence of oxygen with ahydrocarbon substantially free of mercaptans, conducting regeneratedalkali solution from said second regeneration Zone to the second of saiddistillate extraction zones, and

contacting it therein with partially sweetened distillate from said rstextraction Zone, thereby completely sweetening said distillate,separating alkali solution from distillate in said second extractionzone, combining it with the alkali solution in said rst extraction zonefor the removal of mercaptans from said additional amounts of sourdistillate therein, and employing a portion of the completely sweeteneddistillate from said second extraction zone as the mercaptan-freehydrocarbon for extracting the alkali solution in said secondregeneration zone, thereby transferring mercaptans from said solution tosaid sweetened distillate and charging the resulting distillatecontaining mercaptans to said first extraction zone.

DONALD E. PAYNE. VANDERVEER VOORHEES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,336,109 Lowry et al. Dec. 7,1943 2,324,927 Heilman July 20, 1943 2,317,770y Holloway et al Apr. 27,1943 2,285,898 Caselli et al June 9, 1942 2,258,279 Caselli et al Oct.7, 1941 2,337,467 Hewlett Dec. 21, 1943 1,998,863 Chaney et al Apr. 2'3,1935 2,080,564 Craig May 18, 1937 2,001,715 Fischer May 21, 1935i2,369,771 Bond Feb. 20, 1945 2,015,038 Pevere Sept. 17, 1935

